DESCRIPTION: The objective of the proposal is the structural determination of a phosducin/transducin-beta.gamma complex extracted from bovine retina. The components of the complex are involved in the visual signal coupling G-protein in the vertebrate retinal photoreceptor cells. T-beta.gamma is well-known for its role in preventing transducin A from being converted to the active GTP state by rhodopsin. There is now evidence that the availability of T-beta.gamma is regulated by the 28 kDA phosducin which, in its unphosphorylated state, forms a tight complex with T-beta.gamma. The trimeric components of the complex have counterparts in other G protein systems. Thus, the three-dimensional structure of the phosducin/T-beta.gamma complex will not only advance the state-of-knowledge of the visual system at the atomic level, but will be very important for understanding many normal and diseased states which utilize G proteins in signal transduction processes. T-beta is particularly important, not only as the first structural example of analogous proteins in G protein systems which are highly conserved in sequence, but as a model system for a superfamily of proteins which contain a repetitive 40-amino acid sequence characterized by WD and other markers. The size and character of the sequence repeat is unusual and likely to have novel structural features.